This invention relates to field effect transistors and more particularly, to monolithic field effect transistors acting as voltage controlled variable resistors.
The use of a field effect transistor (FET) as a voltage controlled variable resistor is well known. There is a region in the characteristic curve of a FET for small values of the drain to source voltage V.sub.DS, where the drain current I.sub.D varies linearly with V.sub.DS. The V.sub.DS value must be smaller than the gate to source voltage V.sub.GS less the threshold or pinch off voltage V.sub.T, i.e. (V.sub.GS -V.sub.T). The linear relationship between voltage and current allows the FET to be used as a voltage controlled variable resistor. In the linear region, the FET acts as a resistor whose value can be controlled by the gate voltage. The drain can be kept either positive or negative with respect to the source when using the FET in this mode, which gives the FET the bilateral property of an ordinary resistor. The resistance R.sub.DS of the FET in the linear region is given by ##EQU1##
The relationship shows the dependence of the resistance R.sub.DS on V.sub.GS. FIG. 1 is a plot of equation 1 showing the relationship between R.sub.DS and V.sub.GS for a typical single FET designed for use as a voltage controlled resistor. As a negative voltage applied to the gate is increased, the resistance varies substantially linearly between 0.0 and -2.0 volts until pinch-off at approximately -4.0 volts.
The characteristics of individual FETs fabricated in monolithic form are proportional to the gate width and other manufacturing parameters. While the characteristics may vary slightly for individual FETs, the relationship between the resistance and the gate voltage is always in accordance with equation 1. The curve as shown in FIG. 1 may be moved up and down or compressed and expanded, by manufacturing the FET with a gate width that will yield the desired channel resistance. However, the resistance-voltage relationship will remain in accordance with equation 1. In the many applications for voltage controlled variable FET resistors, such as in an automatic gain control circuit, there is often a need for a variable resistor having a voltage versus resistance relationship different than that of equation 1.
An example in the prior art of a monolithic FET used as a voltage variable resistor is described by Fisher et al., "A Linear GaAs MMIC Variable Attenuator", RF Design, October 1987. Fisher teaches the use of three single FETs having a gate width based on a trade off between insertion loss and parasitic capacitance. In addition, complicated external analog control circuitry is required to apply the desired gate voltages to each FET. The control circuitry includes active components requiring D.C. power to drive the circuit.
Another prior art use of a FET resistor is disclosed by Lizama, et al., "1-6 GH.sub.Z GaAs MMIC Linear Attenuator With Integral Drivers", IEEE 1987 Microwave and Millimeter-Wave Monolithic Circuits Symposium. Lizama, et al. uses two FETs in parallel of different gate widths to provide improved linearity of attenuation. Lizama, et al. uses a complex integrated driver circuit consisting of active elements to provide the various gate voltages, that consumes D.C. power and is sensitive to temperature variations and process changes.